1. Field of the Invention
The present invention relates to a process for producing a novel food material, and more particularly to a process for producing from a soybean protein a mass of food material having a texture resembling that of natural meat.
2. Prior Art
The food industry has endeavored to develop low-fat, low-cholesterol food materials from vegetable proteins as substitutes for various meat products. Since natural meat products are generally considered to be composed of muscular fibers of protein, further efforts have been made to obtain food materials having a fibrous or shred-like texture like that of natural meat. A variety of techniques have been developed for producing such food materials. Among the major techniques is a fiber spinning method which is a modification of methods for making synthetic textile fibers. In this method, an alkaline protein solution is extruded through spinnerettes having numerous minute holes of about 0.1 mm in diameter into an acidic coagulating bath which causes precipitation in a filament form. The filaments are cut to length and assembled into a meat analog by use of binding materials (U.S. Pat. Nos. 2,682,466 and 3,482,998).
A currently predominant technique is a modification of methods for foam molding of synthetic resin, namely an extrusion cooking method in which a protein material, water and other food ingredients are heated at elevated temperatures and pressures and forced out in the atmosphere at ambient temperature and pressure from the extruder (thermoplastic extruder), giving an extrudate which has expanded to form a food material having fibrous structure (U.S. Pat. Nos. 3,488,770 and 3,496,858).
On rehydration, the texturized soybean protein thus prepared is given an appearance and a mouthfeel similar to those of cooked meat pieces, and thus can be used as it is. However, the product has only the size of a thumb at largest.
Methods are also known for producing a larger size mass of meat analogs. In the methods, a texturized soybean protein is hydrated, cohered, set and shaped using as a binder an aqueous solution of protein, starch or the like having an adhesive power or gelation properties. Techniques are also known in which a hydrated texturized soybean protein is crushed into hydrated fibrous fragments and the fragments are admixed with about 5 to about 50% by weight, based on the total amount of the materials used, of a binder composed of a gelable protein or starch and other food ingredients and water, whereby the mixture is gelled, set and shaped into meat analog products like hamburger patties, meat chunks, meat loaves, hams, sausages and so forth (U.S. Pat. Nos. 061,784; 4,376,134; 4,495,205 and 4,863,749).
The food materials obtained by the foregoing methods are not homogeneous and thus similar in mouthfeel to meat products, compared with the food materials theretofore prepared by gelling a uniform mixture of water, a gelable protein and/or starch and other food ingredients. However, these food materials do not afford a sufficient degree of mouthfeel like that of meat products, and lack a texture composed of assembled muscular fibers, namely a texture like that of natural meat products. These food materials also fail to yield a feel (proper cohesiveness and brittleness) due to degree of disintegration of food according to the biting force and fail to involve an enhanced level of chewiness (springiness and masticating level).
It is an object of the present invention to provide a process for producing a mass of food material which has considerable size dimensions, good appearance and texture similar to that of natural meat products.
It is another object of the invention to provide a process for producing a food material useful as an all-vegetable, low-calorie, non-cholesterol material.
Other objects and features of the present invention will become more apparent from the following description.
To achieve the foregoing objects, we conducted extensive research and found that when the hydrated fibrous material obtained from soybean protein is concurrently dehydrated and shaped by compression (i.e. subjected to compression molding) and the shaped product is heated and set in a compressed state, the obtained food material is very similar in any of dimensions, appearance, texture and the like to real meat products.
Our additional discovery is as follows. The foregoing process eliminates a need for the animal or plant protein frequently used as a binder and therefore gives a food material entirely free of the fat ingredient contained in the binder. The source of calories and the source of nutritive ingredients can be limited, e.g., by using seasonings and other ingredients limited to those derived from vegetable sources, and a cholesterol-free food material can be obtained. The present invention has been accomplished based on these novel findings.
According to the present invention, there is provided a process for preparing a food material, the process comprising the steps of: concurrently dehydrating and shaping a hydrated fibrous material (HFM) by compression which material is prepared from a soybean protein; and heating the obtained shaped product in a compressed state.
According to the invention, a food material is prepared from the HFM obtained from a soybean protein.
Useful HFM's can be any of those produced by hydrating, swelling and disintegrating the texturized soybean protein (TSP) obtained by the extrusion cooking method, those produced by the fiber spinning method, etc.
Among these HFM's, those obtained from the TSP by the extrusion cooking method are outstanding in mouthfeel and appearance. Discussed below in detail is the method for producing a TSP by the extrusion cooking method.
The TSP is prepared as follows. A mixture of water and soybean proteins as the main ingredient is passed through an extruder at a high temperature and high pressure according to the extrusion cooking method so that the fibers are oriented in a specific direction at a fusing portion of the extruder in the forward end thereof, and a structure with fibers or thin layers oriented in a specific direction is formed in the tissue of TSP. TSP's can be any of those which can be disintegrated into fibrous materials. The water absorbency, fiber properties, flexibility, strength, hardness and other properties of TSP vary with the kind of soybean protein used as the main ingredient, purity of protein, degree of denaturation, amount of protein, kinds and amounts of other protein, starch and the like added when required, amount of water added in the extrusion cooking method, pressurizing and heating conditions in the method, etc. Food materials produced from such TSP have different textures and cooking characteristics.
A preferred soybean protein useful as the main starting material is at least one soybean protein powder selected from the group consisting of soybean flour, defatted soybean flour, concentrated soybean protein powder and isolated soybean protein powder. A suitable soybean protein powder contains at least 50% by weight of soybean protein and has a non-denaturation degree of at least 20, preferably at least 50 in terms of nitrogen solubility index (NSI).
A preferred TSP is one prepared from only the main ingredient, i.e. one or more of the above soybean protein powders. Such TSP can be made into a meat-like protein product free of animal protein. Also, a mixture of soybean protein powder as the main ingredient and an additional ingredient can be used in preparation of TSP. Examples of useful additional ingredients are egg white, milk casein, wheat gluten, proteins derived from plant seeds or the like; starches derived from corn, wheat or like grains or from potatoes, tapioca or the like; etc. The additional ingredient can be used in an amount of not more than 20% by weight, based on the total amount of the main ingredient and the additional ingredient.
The conditions for the extrusion cooking methods are not specifically limited and can be suitably selected from conventional conditions according to the properties of desired TSP.
The TSP having the desired properties can be obtained by adjusting the quantities of starting materials, the conditions for the extrusion cooking method and other factors. Commercially available mass-produced TSP's are also usable.
A TSP suitable for use in the invention has the following characteristics. It has a hydration capacity of about 2 to about 6; it is composed of sufficient fiber tissues; it possesses fibers which, when disintegrated, have a length of at least about 5 mm, preferably about 10 to about 30 mm; and it is in the form of granules, rods, flakes, etc. having a sufficient size to contain such fibers. More specifically, a preferred TSP contains components of at least 10 mm, preferably at least 30 mm, in the entire length as measured along the fibers, in an amount of at least about 75% of the total amount of TSP. The term "hydration capacity" used herein refers to the total amount of water which the TSP is able to hold. The hydration capacity is determined by soaking in excess hot water at 70.degree. C., standing in the water at room temperature for 20 minutes, and draining by further 5 minutes of standing and is calculated by the following equation: EQU hydration capacity=(V-V.sub.0)/V.sub.0
wherein V is the weight of hydrated TSP, and V.sub.0 is the weight of TSP before hydration.
Generally, a particularly suitable TSP is one having a hydration capacity of about 3 to about 6 and prepared from a mixture of isolated soybean protein powder of high protein purity and concentrated soybean protein powder as main ingredients with starch and/or wheat gluten. Such TSP is easy to disintegrate and contains soft, slender fibrous materials. The disintegrated fibrous materials are easy to shape due to the force attributable to intertwining of fibers and can be made into a chicken meat-like food material of relatively high water-content which has a soft texture. The TSP having a hydration capacity of about 2 to about 4 and prepared from defatted soybean protein flour contains fibrous materials which are difficult to disintegrate but proper in chewiness. Such TSP can be formed into a beef-like food material having a strong texture. The food material obtained from a mixture of TSP's having different hydration capacities is irregular in mouthfeel and similar to real meat products, and, for example, gives such mouthfeel as afforded by fatty meat portion.
The TSP is hydrated with hot or warm water to capacity and swollen. The temperature of hot or warm water affects the time required for hydration and swelling and the chewy property of the obtained food material. For example, when the TSP used has a relatively high hydration capacity of about 3 to 6, it becomes hydrated with warm or hot water at about 40 to about 70.degree. C. in about 10 to about 30 minutes, and with hot water at about 70.degree. to about 100.degree. C. in a shorter period. In the latter case, the obtained food material is less chewy than when hydrated with warm or hot water at about 40.degree. to about 70.degree. C. The hydration of TSP having a relatively low hydration capacity of about 2 to about 4 requires about 30 to about 60 minutes when using warm or hot water at about 50.degree. to about 70.degree. C. and a reduced period of about 10 to about 30 minutes when using hot water at about 70.degree. to about 100.degree. C. In the latter case, the obtained food material is less chewy than when hydrated with warm or hot water at about 50.degree. to about 70.degree. C.
A food material having the desired level of chewiness can be obtained by the combination of TSP to be used and a temperature of hot or warm water to be employed for hydration and swelling.
Subsequently the hydrated and swollen TSP is disintegrated by stirring in water. The shape, especially the fiber length, of the hydrated fibrous material obtained by disintegration affects the force by which the fibers are intertwined with one another into a food material, the texture like that of meat, the appearance, etc. Therefore stirring is conducted considering both disintegration and retention of shape. When required, the TSP disintegrated is then washed with fresh water or hot or warm water with stirring to eliminate the undesirable flavor of soybean by replacement therewith. The stirring done for washing as well as for disintegration may achieve an excessive degree of disintegrating effect, which is undesirable because it results in cutting of fibers into shorter pieces.
Stated more specifically, the stirring is effected so that it does not leave the original shape whole of TSP used. A preferred stirring is effected so that 1/3 the whole TSP is reduced to about 1/50 to about 1/10 the original volume, namely to a cross-section area of about 1 to about 50 mm.sup.2, preferably about 5 to about 25 mm.sup.2 and a length of about 5 to about 50 mm, preferably about 10 to about 30 mm. Fine particles may be present as mixed with the disintegrated materials.
Stirring methods effective to disintegrate the TSP include those using a stirrer with agitating blades or a pump with an impeller. The methods comprise soaking the TSP in hot or warm water in such amount that the hydrated and swollen TSP can be freely moved therein, more specifically in an amount of about 7 or more times, preferably about 10 to about 30 times, the weight of the TSP before hydration and subjecting the TSP to impact with the agitating blades or impeller to concurrently achieve vigorous agitation or circulation due to the resulting current.
The stirring time is about 2 to about 30 minutes although variable depending on the type of TSP used as the starting material and the conditions for hydration and swelling. Before stirring, the hydrated and swollen TSP may be softened by compressing or beating to such extent as to avoid fracture. The softening can shorten the stirring time.
In the subsequent washing step to be optionally carried out, a stirring device such as one mentioned above can be used. Means other than stirrers, e.g. conventional washing means, may be used in this step since disintegration is not intended therein. During or after disintegration, the undesirable flavor of soybean is released into fresh water or hot or warm water and eliminated by replacement therewith.
The TSP disintegrated and optionally washed is slightly drained to give a HFM. The draining can be easily done with use of a net, mesh or like screening means such as metal net. For industrial purposes, a rice washer is used which is in the form of an inclined rotating steel net drum equipped with a sprinkler. When a rice washer is used, the disintegrated TSP is pumped, together with water, into the apparatus and the TSP rolls down along the inside of the drum during which the TSP is washed and eventually slightly drained, followed by release of HFM.
The water content of HFM thus obtained is not specifically limited, and is in a range such that the mass of HFM can be charged into a mold or a basket for subsequent dehydration by compression or centrifugation. Usually a proper HFM holds a sufficient amount of water to, e.g. a water content of about 75 to about 95% by weight. The reduction into a smaller size mass as by squeezing should be avoided.
The production of HFM by the fiber spinning method is feasible in the conventional manner. The starting soybean protein can be one usually used in conventional fiber spinning methods. The HFM produced by the method may have the same water content and dimensions as that produced by the extrusion cooking method.
The HFM thus obtained is shaped while being dehydrated by compression. A simple method comprises squeezing the HFM as wrapped up with a water-permeable sheet such as a piece of cotton cloth or a net made of Tetoron (trademark for polyester-type synthetic fiber, product of Toyo Rayon K.K.) and rolling the wrapped HFM with a bamboo screen or the like. In industrial applications, a press or a centrifugal dehydrator may be used. Usually in the press, the HFM is placed on the cavity of the mold and is compressed by the upper and lower plates fixed to the inside of mold as with use of hydraulic means, whereby the dehydration and shaping are concurrently accomplished.
To make the HFM having intertwined fibers into a dehydrated void-free shaped product in the above step, a pressure is applied for at least one minute to such extent that the force ultimately exerted on the plates ranges from about 0.5 to about 20 kg weight/cm.sup.2, preferably about 2 to about 15 kg weight/cm.sup.2. More specifically, for production of a dehydrated shaped product, e.g. about 30 to about 60 mm in thickness, a pressure is gradually raised over a period of about 1 to about 10 minutes.
A desirable shaped product is one having a volume of about 1/2 to about 1/4 that of the HFM placed on the cavity before compression and with a water content of about 60 to about 85% by weight, preferably about 67 to about 80% by weight.
Optionally, a centrifugal dehydrator may be used. For example, in order to produce the same effect as by the press, a vertical batch-type centrifuge is operated by a centrifugal force of about 100 to about 700 G, preferably about 200 to about 500 G, for about 5 to about 30 minutes to achieve dehydration and shaping by compression during which the HFM is pressed against the inner wall of the rotating cylinder to accomplish shaping by compression. When a larger size dehydrated shaped product is produced or when a starting TSP having a high hydration capacity is used, it is desirable to gradually elevate the pressure over an extended time period.
In the above method which simultaneously performs dehydration and shaping, the HFM which is irregularly oriented is oriented only in the centrifugal direction or the pressing direction by the centrifugal force or by the pressing force. When simply compressed with the rolled screen, the HFM is compressed in the centripetal direction and the fiber tissues are oriented in a direction perpendicular to the compressing direction. Such shaped products have fibrous materials not completely but correlatively oriented in one direction so that a cooked product gives a feel due to the orientation of fiber tissues when cut or chewed, and is not incongruous at all. These products are very similar to cooked meat products in texture, appearance, etc.